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<h1>N4JS Language Specification</h1>
<div class="details">
<span id="author" class="author">2019-08-08 13:15:33 CEST</span><br>
<span id="revnumber">version 0.9</span>
</div>
<div id="toc" class="toc2">
<div id="toctitle">Table of Contents</div>
<ul class="sectlevel1">
<li><a href="introduction.html#_introduction">1. Introduction</a>
<ul class="sectlevel2">
<li><a href="introduction.html#_notation">1.1. Notation</a>
<ul class="sectlevel3">
<li><a href="introduction.html#_grammar-notation">1.1.1. Grammar Notation</a></li>
<li><a href="introduction.html#_type-judgments-and-rules-and-constraints-notation">1.1.2. Type Judgments and Rules and Constraints Notation</a>
<ul class="sectlevel4">
<li><a href="introduction.html#_typing-rules-and-judgments">1.1.2.1. Typing Rules and Judgments</a></li>
<li><a href="introduction.html#_types-of-an-element">1.1.2.2. Types of an Element</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="introduction.html#_auxiliary-functions">1.2. Auxiliary Functions</a>
<ul class="sectlevel3">
<li><a href="introduction.html#_binding">1.2.1. Binding</a></li>
<li><a href="introduction.html#_merging-types">1.2.2. Merging Types</a>
<ul class="sectlevel4">
<li><a href="introduction.html#_logic-formulas">1.2.2.1. Logic Formulas</a></li>
</ul>
</li>
<li><a href="introduction.html#_symbols-and-font-convention">1.2.3. Symbols and Font Convention</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="grammar.html#_grammar">2. Grammar</a>
<ul class="sectlevel2">
<li><a href="grammar.html#_lexical-conventions">2.1. Lexical Conventions</a>
<ul class="sectlevel3">
<li><a href="grammar.html#_identifier-names-and-identifiers">2.1.1. Identifier Names and Identifiers</a></li>
<li><a href="grammar.html#_this-keyword">2.1.2. This Keyword</a></li>
<li><a href="grammar.html#_regular-expression-literals">2.1.3. Regular Expression Literals</a></li>
<li><a href="grammar.html#_automatic-semicolon-insertion">2.1.4. Automatic Semicolon Insertion</a></li>
<li><a href="grammar.html#_jsdoc">2.1.5. JSDoc</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="names.html#_names">3. Names</a>
<ul class="sectlevel2">
<li><a href="names.html#_access-control">3.1. Access Control</a></li>
<li><a href="names.html#_accessibility-of-types-top-level-variables-and-function-declarations">3.2. Accessibility of Types, Top-Level Variables and Function Declarations</a>
<ul class="sectlevel3">
<li><a href="names.html#_accessibility-of-members">3.2.1. Accessibility of Members</a></li>
<li><a href="names.html#_valid-names">3.2.2. Valid Names</a></li>
<li><a href="names.html#_qualified-names">3.2.3. Qualified Names</a></li>
<li><a href="names.html#_name-duplicates">3.2.4. Name Duplicates</a>
<ul class="sectlevel4">
<li><a href="names.html#_lexical-environment">3.2.4.1. Lexical Environment</a></li>
<li><a href="names.html#_duplicates-and-shadowing">3.2.4.2. Duplicates and Shadowing</a></li>
</ul>
</li>
</ul>
</li>
</ul>
</li>
<li><a href="types.html#_types">4. Types</a>
<ul class="sectlevel2">
<li><a href="types.html#_overview">4.1. Overview</a></li>
<li><a href="types.html#_type-expressions">4.2. Type Expressions</a>
<ul class="sectlevel3">
<li><a href="types.html#_syntax">4.2.1. Syntax</a></li>
<li><a href="types.html#_properties">4.2.2. Properties</a></li>
<li><a href="types.html#_semantics">4.2.3. Semantics</a></li>
</ul>
</li>
<li><a href="types.html#_type-inference">4.3. Type Inference</a></li>
<li><a href="types.html#_generic-and-parameterized-types">4.4. Generic and Parameterized Types</a>
<ul class="sectlevel3">
<li><a href="types.html#_generic-types">4.4.1. Generic Types</a></li>
<li><a href="types.html#_type-variables">4.4.2. Type Variables</a></li>
<li><a href="types.html#_parameterized-types">4.4.3. Parameterized Types</a></li>
</ul>
</li>
<li><a href="types.html#_primitive-ecmascript-types">4.5. Primitive ECMAScript Types</a>
<ul class="sectlevel3">
<li><a href="types.html#_undefined-type">4.5.1. Undefined Type</a></li>
<li><a href="types.html#_null-type">4.5.2. Null Type</a></li>
<li><a href="types.html#_primitive-boolean-type">4.5.3. Primitive Boolean Type</a></li>
<li><a href="types.html#_primitive-string-type">4.5.4. Primitive String Type</a></li>
<li><a href="types.html#_primitive-number-type">4.5.5. Primitive Number Type</a></li>
<li><a href="types.html#_primitive-type-int">4.5.6. Primitive Type int</a></li>
<li><a href="types.html#_primitive-symbol-type">4.5.7. Primitive Symbol Type</a></li>
</ul>
</li>
<li><a href="types.html#_primitive-n4js-types">4.6. Primitive N4JS Types</a>
<ul class="sectlevel3">
<li><a href="types.html#_any-type">4.6.1. Any Type</a>
<ul class="sectlevel4">
<li><a href="types.html#any-type-semantics">4.6.1.1. Semantics</a></li>
<li><a href="types.html#any-type-type-inference">4.6.1.2. Type Inference</a>
<ul class="sectlevel5">
<li><a href="types.html#_default-type-of-variables">4.6.1.2.1. Default Type of Variables</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="types.html#_void-type">4.6.2. Void Type</a>
<ul class="sectlevel4">
<li><a href="types.html#void-type-semantics">4.6.2.1. Semantics</a></li>
</ul>
</li>
<li><a href="types.html#_unknown-type">4.6.3. Unknown Type</a></li>
<li><a href="types.html#_primitive-pathselector-and-i18nkey">4.6.4. Primitive Pathselector and I18nKey</a>
<ul class="sectlevel4">
<li><a href="types.html#pathselector-semantics">4.6.4.1. Semantics</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="types.html#_built-in-ecmascript-object-types">4.7. Built-in ECMAScript Object Types</a>
<ul class="sectlevel3">
<li><a href="types.html#ECMAScript-objects-semantics">4.7.1. Semantics</a></li>
<li><a href="types.html#_object-type">4.7.2. Object Type</a></li>
<li><a href="types.html#_function-object-type">4.7.3. Function-Object-Type</a></li>
<li><a href="types.html#_array-object-type">4.7.4. Array Object Type</a></li>
<li><a href="types.html#_string-object-type">4.7.5. String Object Type</a></li>
<li><a href="types.html#_boolean-object-type">4.7.6. Boolean Object Type</a></li>
<li><a href="types.html#_number-object-type">4.7.7. Number Object Type</a></li>
<li><a href="types.html#_global-object-type">4.7.8. Global Object Type</a></li>
<li><a href="types.html#_symbol">4.7.9. Symbol</a></li>
<li><a href="types.html#_promise">4.7.10. Promise</a></li>
<li><a href="types.html#_iterator-interface">4.7.11. Iterator Interface</a></li>
<li><a href="types.html#_iterable-interface">4.7.12. Iterable Interface</a></li>
</ul>
</li>
<li><a href="types.html#_built-in-n4js-types">4.8. Built-In N4JS Types</a>
<ul class="sectlevel3">
<li><a href="types.html#_n4object">4.8.1. N4Object</a>
<ul class="sectlevel4">
<li><a href="types.html#N4Object-semantics">4.8.1.1. Semantics</a></li>
</ul>
</li>
<li><a href="types.html#_n4class">4.8.2. N4Class</a></li>
<li><a href="types.html#IterableN">4.8.3. IterableN</a></li>
</ul>
</li>
<li><a href="types.html#_type-modifiers">4.9. Type Modifiers</a>
<ul class="sectlevel3">
<li><a href="types.html#Type_Modifiers_Dynamic">4.9.1. Dynamic</a></li>
<li><a href="types.html#_optional-return-types">4.9.2. Optional Return Types</a></li>
</ul>
</li>
<li><a href="types.html#_union-and-intersection-type-composed-types">4.10. Union and Intersection Type (Composed Types)</a>
<ul class="sectlevel3">
<li><a href="types.html#_union-type">4.10.1. Union Type</a>
<ul class="sectlevel4">
<li><a href="types.html#union-type-syntax">4.10.1.1. Syntax</a></li>
<li><a href="types.html#union-type-semantics">4.10.1.2. Semantics</a></li>
<li><a href="types.html#_warnings">4.10.1.3. Warnings</a></li>
</ul>
</li>
<li><a href="types.html#_intersection-type">4.10.2. Intersection Type</a>
<ul class="sectlevel4">
<li><a href="types.html#intersection-type-syntax">4.10.2.1. Syntax</a></li>
<li><a href="types.html#intersection-type-semantics">4.10.2.2. Semantics</a></li>
<li><a href="types.html#_warnings-2">4.10.2.3. Warnings</a></li>
</ul>
</li>
<li><a href="types.html#_composed-types-in-wildcards">4.10.3. Composed Types in Wildcards</a></li>
<li><a href="types.html#_property-access-for-composed-types">4.10.4. Property Access for Composed Types</a>
<ul class="sectlevel4">
<li><a href="types.html#_properties-of-union-type">4.10.4.1. Properties of Union Type</a>
<ul class="sectlevel5">
<li><a href="types.html#_remarks-on-union-type-s-members">4.10.4.1.1. Remarks on union type’s members:</a></li>
</ul>
</li>
<li><a href="types.html#_properties-of-intersection-type">4.10.4.2. Properties of Intersection Type</a>
<ul class="sectlevel5">
<li><a href="types.html#_remarks-on-intersection-type-s-methods">4.10.4.2.1. Remarks on intersection type’s methods:</a></li>
</ul>
</li>
</ul>
</li>
</ul>
</li>
<li><a href="types.html#_constructor-and-classifier-type">4.11. Constructor and Classifier Type</a>
<ul class="sectlevel3">
<li><a href="types.html#_syntax-3">4.11.1. Syntax</a></li>
<li><a href="types.html#_semantics-2">4.11.2. Semantics</a></li>
<li><a href="types.html#_constructors-and-prototypes-in-ecmascript-2015">4.11.3. Constructors and Prototypes in ECMAScript 2015</a></li>
</ul>
</li>
<li><a href="types.html#_this-type">4.12. This Type</a>
<ul class="sectlevel3">
<li><a href="types.html#this-type-syntax">4.12.1. Syntax</a></li>
<li><a href="types.html#this-keyword-semantics">4.12.2. Semantics</a></li>
</ul>
</li>
<li><a href="types.html#_enums">4.13. Enums</a>
<ul class="sectlevel3">
<li><a href="types.html#_enums-n4js">4.13.1. Enums (N4JS)</a>
<ul class="sectlevel4">
<li><a href="types.html#enums-syntax">4.13.1.1. Syntax</a></li>
<li><a href="types.html#enums-semantics">4.13.1.2. Semantics</a></li>
</ul>
</li>
<li><a href="types.html#_string-based-enums">4.13.2. String-Based Enums</a></li>
</ul>
</li>
<li><a href="types.html#_short-hand-syntax">4.14. Short-Hand Syntax</a>
<ul class="sectlevel3">
<li><a href="types.html#_array-short-hand-syntax">4.14.1. Array Short-Hand Syntax</a></li>
<li><a href="types.html#_iterablen-short-hand-syntax">4.14.2. IterableN Short-Hand Syntax</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="classifiers.html#_classifiers">5. Classifiers</a>
<ul class="sectlevel2">
<li><a href="classifiers.html#_n4js-specific-classifiers">5.1. N4JS Specific Classifiers</a>
<ul class="sectlevel3">
<li><a href="classifiers.html#_properties-2">5.1.1. Properties</a></li>
<li><a href="classifiers.html#_common-semantics-of-classifiers">5.1.2. Common Semantics of Classifiers</a></li>
<li><a href="classifiers.html#_classes">5.1.3. Classes</a>
<ul class="sectlevel4">
<li><a href="classifiers.html#_definition-of-classes">5.1.3.1. Definition of Classes</a>
<ul class="sectlevel5">
<li><a href="classifiers.html#class-syntax">5.1.3.1.1. Syntax</a></li>
<li><a href="classifiers.html#class-properties">5.1.3.1.2. Properties</a></li>
<li><a href="classifiers.html#class-type-inference">5.1.3.1.3. Type Inference</a></li>
</ul>
</li>
<li><a href="classifiers.html#class-semantics">5.1.3.2. Semantics</a></li>
<li><a href="classifiers.html#_final-modifier">5.1.3.3. Final Modifier</a></li>
<li><a href="classifiers.html#_abstract-classes">5.1.3.4. Abstract Classes</a></li>
<li><a href="classifiers.html#_non-instantiable-classes">5.1.3.5. Non-Instantiable Classes</a></li>
<li><a href="classifiers.html#_superclass">5.1.3.6. Superclass</a></li>
</ul>
</li>
<li><a href="classifiers.html#_interfaces">5.1.4. Interfaces</a>
<ul class="sectlevel4">
<li><a href="classifiers.html#_definition-of-interfaces">5.1.4.1. Definition of Interfaces</a>
<ul class="sectlevel5">
<li><a href="classifiers.html#interfaces-syntax">5.1.4.1.1. Syntax</a></li>
<li><a href="classifiers.html#interfaces-properties">5.1.4.1.2. Properties</a></li>
<li><a href="classifiers.html#interfaces-type-inference">5.1.4.1.3. Type Inference</a></li>
<li><a href="classifiers.html#interfaces-semantics">5.1.4.1.4. Semantics</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="classifiers.html#_generic-classifiers">5.1.5. Generic Classifiers</a></li>
<li><a href="classifiers.html#sec:definition-site-variance">5.1.6. Definition-Site Variance</a></li>
</ul>
</li>
<li><a href="classifiers.html#_members">5.2. Members</a>
<ul class="sectlevel3">
<li><a href="classifiers.html#_syntax-4">5.2.1. Syntax</a>
<ul class="sectlevel4">
<li><a href="classifiers.html#_properties-3">5.2.1.1. Properties</a></li>
</ul>
</li>
<li><a href="classifiers.html#_semantics-3">5.2.2. Semantics</a></li>
<li><a href="classifiers.html#_methods">5.2.3. Methods</a>
<ul class="sectlevel4">
<li><a href="classifiers.html#_syntax-5">5.2.3.1. Syntax</a></li>
<li><a href="classifiers.html#_properties-4">5.2.3.2. Properties</a></li>
<li><a href="classifiers.html#_semantics-4">5.2.3.3. Semantics</a></li>
<li><a href="classifiers.html#_final-methods">5.2.3.4. Final Methods</a></li>
<li><a href="classifiers.html#_abstract-methods">5.2.3.5. Abstract Methods</a></li>
<li><a href="classifiers.html#_generic-methods">5.2.3.6. Generic Methods</a></li>
</ul>
</li>
<li><a href="classifiers.html#_default-methods-in-interfaces">5.2.4. Default Methods in Interfaces</a>
<ul class="sectlevel4">
<li><a href="classifiers.html#_asynchronous-methods">5.2.4.1. Asynchronous Methods</a></li>
</ul>
</li>
<li><a href="classifiers.html#_constructors">5.2.5. Constructors</a>
<ul class="sectlevel4">
<li><a href="classifiers.html#_structural-this-type-in-constructor">5.2.5.1. Structural This Type in Constructor</a></li>
<li><a href="classifiers.html#spec-constructor">5.2.5.2. @Spec Constructor</a></li>
<li><a href="classifiers.html#_callable-constructors">5.2.5.3. Callable Constructors</a></li>
<li><a href="classifiers.html#_covariant-constructors">5.2.5.4. Covariant Constructors</a></li>
</ul>
</li>
<li><a href="classifiers.html#_data-fields">5.2.6. Data Fields</a>
<ul class="sectlevel4">
<li><a href="classifiers.html#data-fields-syntax">5.2.6.1. Syntax</a></li>
<li><a href="classifiers.html#data-fields-properties">5.2.6.2. Properties</a>
<ul class="sectlevel5">
<li><a href="classifiers.html#data-fields-semantics">5.2.6.2.1. Semantics</a></li>
<li><a href="classifiers.html#data-fields-type-inference">5.2.6.2.2. Type Inference</a></li>
</ul>
</li>
<li><a href="classifiers.html#_assignment-modifiers">5.2.6.3. Assignment Modifiers</a></li>
<li><a href="classifiers.html#_field-accessors-getter-setter">5.2.6.4. Field Accessors (Getter/Setter)</a>
<ul class="sectlevel5">
<li><a href="classifiers.html#field-acessors-syntax">5.2.6.4.1. Syntax</a></li>
<li><a href="classifiers.html#field-acessors-properties">5.2.6.4.2. Properties</a></li>
<li><a href="classifiers.html#field-accessors-semantics">5.2.6.4.3. Semantics</a></li>
</ul>
</li>
<li><a href="classifiers.html#optional-fields">5.2.6.5. Optional Fields</a>
<ul class="sectlevel5">
<li><a href="classifiers.html#_syntax-6">5.2.6.5.1. Syntax</a></li>
<li><a href="classifiers.html#_semantics-5">5.2.6.5.2. Semantics</a></li>
<li><a href="classifiers.html#_background">5.2.6.5.3. Background</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="classifiers.html#_static-members">5.2.7. Static Members</a>
<ul class="sectlevel4">
<li><a href="classifiers.html#_access-from-and-to-static-members">5.2.7.1. Access From and To Static Members</a></li>
<li><a href="classifiers.html#_generic-static-methods">5.2.7.2. Generic static methods</a></li>
<li><a href="classifiers.html#_static-members-of-interfaces">5.2.7.3. Static Members of Interfaces</a></li>
</ul>
</li>
<li><a href="classifiers.html#_redefinition-of-members">5.2.8. Redefinition of Members</a>
<ul class="sectlevel4">
<li><a href="classifiers.html#_overriding-of-members">5.2.8.1. Overriding of Members</a></li>
<li><a href="classifiers.html#_implementation-of-members">5.2.8.2. Implementation of Members</a>
<ul class="sectlevel5">
<li><a href="classifiers.html#_member-consumption">5.2.8.2.1. Member Consumption</a></li>
<li><a href="classifiers.html#_member-implementation">5.2.8.2.2. Member Implementation</a></li>
</ul>
</li>
</ul>
</li>
</ul>
</li>
<li><a href="classifiers.html#_structural-typing">5.3. Structural Typing</a>
<ul class="sectlevel3">
<li><a href="classifiers.html#_syntax-7">5.3.1. Syntax</a></li>
<li><a href="classifiers.html#_definition-site-structural-typing">5.3.2. Definition Site Structural Typing</a></li>
<li><a href="classifiers.html#_use-site-structural-typing">5.3.3. Use-Site Structural Typing</a></li>
<li><a href="classifiers.html#structural-readWriteInit-field-typing">5.3.4. Structural Read-only, Write-only and Initializer Field Typing</a></li>
<li><a href="classifiers.html#_public-setter-annotated-with-code-providesinitializer-code">5.3.5. Public Setter Annotated With <code>ProvidesInitializer</code></a></li>
<li><a href="classifiers.html#_structural-types-with-optional-fields">5.3.6. Structural Types With Optional Fields</a></li>
<li><a href="classifiers.html#_structural-types-with-access-modifier">5.3.7. Structural Types With Access Modifier</a></li>
<li><a href="classifiers.html#_structural-types-with-additional-members">5.3.8. Structural Types With Additional Members</a>
<ul class="sectlevel4">
<li><a href="classifiers.html#_syntax-8">5.3.8.1. Syntax</a>
<ul class="sectlevel5">
<li><a href="classifiers.html#_semantics-6">5.3.8.1.1. Semantics</a></li>
</ul>
</li>
</ul>
</li>
</ul>
</li>
</ul>
</li>
<li><a href="functions.html#_functions">6. Functions</a>
<ul class="sectlevel2">
<li><a href="functions.html#_function-type">6.1. Function Type</a>
<ul class="sectlevel3">
<li><a href="functions.html#_properties-5">6.1.1. Properties</a></li>
<li><a href="functions.html#function-type-inference">6.1.2. Type Inference</a></li>
<li><a href="functions.html#_autoboxing-of-function-type">6.1.3. Autoboxing of Function Type</a></li>
<li><a href="functions.html#_arguments-object">6.1.4. Arguments Object</a></li>
</ul>
</li>
<li><a href="functions.html#_ecmascript-5-function-definition">6.2. ECMAScript 5 Function Definition</a>
<ul class="sectlevel3">
<li><a href="functions.html#_function-declaration">6.2.1. Function Declaration</a>
<ul class="sectlevel4">
<li><a href="functions.html#_syntax-9">6.2.1.1. Syntax</a></li>
<li><a href="functions.html#_semantics-8">6.2.1.2. Semantics</a></li>
</ul>
</li>
<li><a href="functions.html#_function-expression">6.2.2. Function Expression</a>
<ul class="sectlevel4">
<li><a href="functions.html#function-expression-syntax">6.2.2.1. Syntax</a></li>
<li><a href="functions.html#_semantics-and-type-inference">6.2.2.2. Semantics and Type Inference</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="functions.html#_ecmascript-2015-function-definition">6.3. ECMAScript 2015 Function Definition</a>
<ul class="sectlevel3">
<li><a href="functions.html#_formal-parameters">6.3.1. Formal Parameters</a>
<ul class="sectlevel4">
<li><a href="functions.html#Type_Modifiers_Optional">6.3.1.1. Optional Parameters</a></li>
<li><a href="functions.html#Type_Modifiers_Default">6.3.1.2. Default Parameters</a></li>
<li><a href="functions.html#Type_Modifiers_Variadic">6.3.1.3. Variadic</a></li>
</ul>
</li>
<li><a href="functions.html#_generator-functions">6.3.2. Generator Functions</a>
<ul class="sectlevel4">
<li><a href="functions.html#generator-functions-syntax">6.3.2.1. Syntax</a></li>
<li><a href="functions.html#generator-functions-semantics">6.3.2.2. Semantics</a></li>
<li><a href="functions.html#_generator-arrow-functions">6.3.2.3. Generator Arrow Functions</a></li>
</ul>
</li>
<li><a href="functions.html#_arrow-function-expression">6.3.3. Arrow Function Expression</a>
<ul class="sectlevel4">
<li><a href="functions.html#arrow-function-expression-syntax">6.3.3.1. Syntax</a></li>
<li><a href="functions.html#arrow-function-expression-semantics-and-type-inference">6.3.3.2. Semantics and Type Inference</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="functions.html#_ecmascript-proposals-function-definition">6.4. ECMAScript Proposals Function Definition</a>
<ul class="sectlevel3">
<li><a href="functions.html#_asynchronous-functions">6.4.1. Asynchronous Functions</a>
<ul class="sectlevel4">
<li><a href="functions.html#asynchronous-functions-syntax">6.4.1.1. Syntax</a></li>
<li><a href="functions.html#asynchronous-functions-semantics">6.4.1.2. Semantics</a></li>
<li><a href="functions.html#_asynchronous-arrow-functions">6.4.1.3. Asynchronous Arrow Functions</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="functions.html#_n4js-extended-function-definition">6.5. N4JS Extended Function Definition</a>
<ul class="sectlevel3">
<li><a href="functions.html#_generic-functions">6.5.1. Generic Functions</a></li>
<li><a href="functions.html#_promisifiable-functions">6.5.2. Promisifiable Functions</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="conversions_and_reflection.html#_conversions-and-reflection">7. Conversions and Reflection</a>
<ul class="sectlevel2">
<li><a href="conversions_and_reflection.html#_autoboxing-and-coercing">7.1. Autoboxing and Coercing</a>
<ul class="sectlevel3">
<li><a href="conversions_and_reflection.html#_coercing">7.1.1. Coercing</a></li>
<li><a href="conversions_and_reflection.html#_autoboxing-of-primitives">7.1.2. Autoboxing of Primitives</a></li>
<li><a href="conversions_and_reflection.html#_autoboxing-of-function-expressions-and-declarations">7.1.3. Autoboxing of Function Expressions and Declarations</a></li>
</ul>
</li>
<li><a href="conversions_and_reflection.html#_auto-conversion-of-objects">7.2. Auto-Conversion of Objects</a>
<ul class="sectlevel3">
<li><a href="conversions_and_reflection.html#_auto-conversion-of-class-instances">7.2.1. Auto-Conversion of Class Instances</a>
<ul class="sectlevel4">
<li><a href="conversions_and_reflection.html#_auto-conversion-of-interface-instances">7.2.1.1. Auto-Conversion of Interface Instances</a></li>
</ul>
</li>
<li><a href="conversions_and_reflection.html#_auto-conversion-of-enum-literals">7.2.2. Auto-Conversion of Enum Literals</a></li>
</ul>
</li>
<li><a href="conversions_and_reflection.html#_type-cast-and-type-check">7.3. Type Cast and Type Check</a>
<ul class="sectlevel3">
<li><a href="conversions_and_reflection.html#_type-cast">7.3.1. Type Cast</a></li>
<li><a href="conversions_and_reflection.html#_type-check">7.3.2. Type Check</a></li>
</ul>
</li>
<li><a href="conversions_and_reflection.html#_reflection-meta-information">7.4. Reflection meta-information</a>
<ul class="sectlevel3">
<li><a href="conversions_and_reflection.html#_reflection-for-classes">7.4.1. Reflection for Classes</a></li>
<li><a href="conversions_and_reflection.html#_reflection-for-interfaces">7.4.2. Reflection for Interfaces</a></li>
<li><a href="conversions_and_reflection.html#_reflection-for-enumerations">7.4.3. Reflection for Enumerations</a></li>
</ul>
</li>
<li><a href="conversions_and_reflection.html#_conversion-of-primitive-types">7.5. Conversion of primitive types</a></li>
</ul>
</li>
<li><a href="expressions.html#_expressions">8. Expressions</a>
<ul class="sectlevel2">
<li><a href="expressions.html#_ecmascript-5-expressions">8.1. ECMAScript 5 Expressions</a>
<ul class="sectlevel3">
<li><a href="expressions.html#_the-this-literal">8.1.1. The this Literal</a></li>
<li><a href="expressions.html#_identifier">8.1.2. Identifier</a></li>
<li><a href="expressions.html#_literals">8.1.3. Literals</a>
<ul class="sectlevel4">
<li><a href="expressions.html#_integer-literals">8.1.3.1. Integer Literals</a></li>
</ul>
</li>
<li><a href="expressions.html#_array-literal">8.1.4. Array Literal</a></li>
<li><a href="expressions.html#_object-literal">8.1.5. Object Literal</a>
<ul class="sectlevel4">
<li><a href="expressions.html#_properties-6">8.1.5.1. Properties</a></li>
<li><a href="expressions.html#_scoping-and-linking">8.1.5.2. Scoping and linking</a></li>
</ul>
</li>
<li><a href="expressions.html#_parenthesized-expression-and-grouping-operator">8.1.6. Parenthesized Expression and Grouping Operator</a></li>
<li><a href="expressions.html#_property-accessors">8.1.7. Property Accessors</a>
<ul class="sectlevel4">
<li><a href="expressions.html#properties-1">8.1.7.1. Properties</a></li>
</ul>
</li>
<li><a href="expressions.html#_new-expression">8.1.8. New Expression</a></li>
<li><a href="expressions.html#_function-expression-2">8.1.9. Function Expression</a></li>
<li><a href="expressions.html#_function-calls">8.1.10. Function Calls</a></li>
<li><a href="expressions.html#_postfix-expression">8.1.11. Postfix Expression</a></li>
<li><a href="expressions.html#_unary-expression">8.1.12. Unary Expression</a></li>
<li><a href="expressions.html#_multiplicative-expression">8.1.13. Multiplicative Expression</a></li>
<li><a href="expressions.html#_additive-expression">8.1.14. Additive Expression</a>
<ul class="sectlevel4">
<li><a href="expressions.html#type-inference-10">8.1.14.1. Type Inference</a></li>
</ul>
</li>
<li><a href="expressions.html#_bitwise-shift-expression">8.1.15. Bitwise Shift Expression</a></li>
<li><a href="expressions.html#_relational-expression">8.1.16. Relational Expression</a></li>
<li><a href="expressions.html#_equality-expression">8.1.17. Equality Expression</a></li>
<li><a href="expressions.html#_binary-bitwise-expression">8.1.18. Binary Bitwise Expression</a></li>
<li><a href="expressions.html#_binary-logical-expression">8.1.19. Binary Logical Expression</a></li>
<li><a href="expressions.html#_conditional-expression">8.1.20. Conditional Expression</a></li>
<li><a href="expressions.html#_assignment-expression">8.1.21. Assignment Expression</a></li>
<li><a href="expressions.html#_comma-expression">8.1.22. Comma Expression</a></li>
</ul>
</li>
<li><a href="expressions.html#_ecmascript-6-expressions">8.2. ECMAScript 6 Expressions</a>
<ul class="sectlevel3">
<li><a href="expressions.html#_the-super-keyword">8.2.1. The super Keyword</a></li>
</ul>
</li>
<li><a href="expressions.html#_ecmascript-7-expressions">8.3. ECMAScript 7 Expressions</a>
<ul class="sectlevel3">
<li><a href="expressions.html#_await-expression">8.3.1. Await Expression</a></li>
</ul>
</li>
<li><a href="expressions.html#_n4js-specific-expressions">8.4. N4JS Specific Expressions</a>
<ul class="sectlevel3">
<li><a href="expressions.html#_class-expression">8.4.1. Class Expression</a></li>
<li><a href="expressions.html#_cast-as-expression">8.4.2. Cast (As) Expression</a>
<ul class="sectlevel4">
<li><a href="expressions.html#cast-as-expression-semantics-type-inference">8.4.2.1. Semantics and Type Inference</a></li>
</ul>
</li>
<li><a href="expressions.html#Import_Calls">8.4.3. Import Calls</a></li>
</ul>
</li>
<li><a href="expressions.html#compile-time-expressions">8.5. Compile-Time Expressions</a></li>
</ul>
</li>
<li><a href="statements.html#_statements">9. Statements</a>
<ul class="sectlevel2">
<li><a href="statements.html#_ecmascript-5-statements">9.1. ECMAScript 5 Statements</a>
<ul class="sectlevel3">
<li><a href="statements.html#_function-or-field-accessor-bodies">9.1.1. Function or Field Accessor Bodies</a></li>
<li><a href="statements.html#_variable-statement">9.1.2. Variable Statement</a></li>
<li><a href="statements.html#_if-statement">9.1.3. If Statement</a></li>
<li><a href="statements.html#_iteration-statements">9.1.4. Iteration Statements</a></li>
<li><a href="statements.html#_return-statement">9.1.5. Return Statement</a></li>
<li><a href="statements.html#_with-statement">9.1.6. With Statement</a></li>
<li><a href="statements.html#_switch-statement">9.1.7. Switch Statement</a></li>
<li><a href="statements.html#_throw-try-and-catch-statements">9.1.8. Throw, Try, and Catch Statements</a></li>
<li><a href="statements.html#_debugger-statement">9.1.9. Debugger Statement</a></li>
</ul>
</li>
<li><a href="statements.html#_ecmascript-6-statements">9.2. ECMAScript 6 Statements</a>
<ul class="sectlevel3">
<li><a href="statements.html#_let">9.2.1. Let</a></li>
<li><a href="statements.html#_const">9.2.2. Const</a></li>
<li><a href="statements.html#_for-of-statement">9.2.3. <code>for &#8230;&#8203; of</code> statement</a></li>
<li><a href="statements.html#_import-statement">9.2.4. Import Statement</a>
<ul class="sectlevel4">
<li><a href="statements.html#Dynamic_Imports">9.2.4.1. Dynamic Imports</a></li>
<li><a href="statements.html#_immutabilaty-of-imports">9.2.4.2. Immutabilaty of Imports</a></li>
</ul>
</li>
<li><a href="statements.html#_export-statement">9.2.5. Export Statement</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="annotations.html#_annotations">10. Annotations</a>
<ul class="sectlevel2">
<li><a href="annotations.html#_introduction-2">10.1. Introduction</a>
<ul class="sectlevel3">
<li><a href="annotations.html#_syntax-13">10.1.1. Syntax</a></li>
<li><a href="annotations.html#_properties-7">10.1.2. Properties</a></li>
<li><a href="annotations.html#_element-specific-annotations">10.1.3. Element-Specific Annotations</a></li>
<li><a href="annotations.html#_general-annotations">10.1.4. General Annotations</a>
<ul class="sectlevel4">
<li><a href="annotations.html#_idebug">10.1.4.1. IDEBUG</a></li>
</ul>
</li>
<li><a href="annotations.html#idebug-syntax">10.1.5. Syntax</a>
<ul class="sectlevel4">
<li><a href="annotations.html#_semantics-11">10.1.5.1. Semantics</a></li>
<li><a href="annotations.html#_suppress-warnings">10.1.5.2. Suppress Warnings</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="annotations.html#_declaration-of-annotations">10.2. Declaration of Annotations</a></li>
</ul>
</li>
<li><a href="extended_fetaures.html#_extended-fetaures">11. Extended Fetaures</a>
<ul class="sectlevel2">
<li><a href="extended_fetaures.html#_array-and-object-destructuring">11.1. Array and Object Destructuring</a>
<ul class="sectlevel3">
<li><a href="extended_fetaures.html#_syntax-14">11.1.1. Syntax</a></li>
<li><a href="extended_fetaures.html#_semantics-12">11.1.2. Semantics</a></li>
</ul>
</li>
<li><a href="extended_fetaures.html#_dependency-injection">11.2. Dependency Injection</a>
<ul class="sectlevel3">
<li><a href="extended_fetaures.html#_di-components-and-injectors">11.2.1. DI Components and Injectors</a>
<ul class="sectlevel4">
<li><a href="extended_fetaures.html#_dicomponent-relations">11.2.1.1. DIComponent Relations</a></li>
</ul>
</li>
<li><a href="extended_fetaures.html#_binders-and-bindings">11.2.2. Binders and Bindings</a></li>
<li><a href="extended_fetaures.html#_injection-points">11.2.3. Injection Points</a>
<ul class="sectlevel4">
<li><a href="extended_fetaures.html#_field-injection">11.2.3.1. Field Injection</a></li>
<li><a href="extended_fetaures.html#_constructor-injection">11.2.3.2. Constructor Injection</a></li>
<li><a href="extended_fetaures.html#_method-injection">11.2.3.3. Method Injection</a>
<ul class="sectlevel5">
<li><a href="extended_fetaures.html#_provider">11.2.3.3.1. Provider</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="extended_fetaures.html#_n4js-di-life-cycle-and-scopes">11.2.4. N4JS DI Life Cycle and Scopes</a>
<ul class="sectlevel4">
<li><a href="extended_fetaures.html#_injection-cylces">11.2.4.1. Injection Cylces</a></li>
<li><a href="extended_fetaures.html#_default-scope">11.2.4.2. Default Scope</a></li>
<li><a href="extended_fetaures.html#_singleton-scope">11.2.4.3. Singleton Scope</a></li>
<li><a href="extended_fetaures.html#_per-injection-chain-singleton">11.2.4.4. Per Injection Chain Singleton</a></li>
</ul>
</li>
<li><a href="extended_fetaures.html#_validation-of-callsites-targeting-n4injector-methods">11.2.5. Validation of callsites targeting N4Injector methods</a></li>
<li><a href="extended_fetaures.html#_n4js-di-annotations">11.2.6. N4JS DI Annotations</a>
<ul class="sectlevel4">
<li><a href="extended_fetaures.html#_n4js-di-generateinjector">11.2.6.1. N4JS DI @GenerateInjector</a></li>
<li><a href="extended_fetaures.html#_n4js-di-withparentinjector">11.2.6.2. N4JS DI @WithParentInjector</a></li>
<li><a href="extended_fetaures.html#_n4js-di-usebinder">11.2.6.3. N4JS DI @UseBinder</a></li>
<li><a href="extended_fetaures.html#_n4js-di-binder">11.2.6.4. N4JS DI @Binder</a></li>
<li><a href="extended_fetaures.html#_n4js-di-bind">11.2.6.5. N4JS DI @Bind</a></li>
<li><a href="extended_fetaures.html#_n4js-di-provides">11.2.6.6. N4JS DI @Provides</a></li>
<li><a href="extended_fetaures.html#_n4js-di-inject">11.2.6.7. N4JS DI @Inject</a></li>
<li><a href="extended_fetaures.html#_n4js-di-singleton">11.2.6.8. N4JS DI @Singleton</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="extended_fetaures.html#_test-support">11.3. Test Support</a></li>
<li><a href="extended_fetaures.html#_polyfill-definitions">11.4. Polyfill Definitions</a>
<ul class="sectlevel3">
<li><a href="extended_fetaures.html#_runtime-polyfill-definitions">11.4.1. Runtime Polyfill Definitions</a></li>
<li><a href="extended_fetaures.html#_static-polyfill-definitions">11.4.2. Static Polyfill Definitions</a></li>
<li><a href="extended_fetaures.html#_transpiling-static-polyfilled-classes">11.4.3. Transpiling static polyfilled classes</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="components.html#_components">12. Components</a>
<ul class="sectlevel2">
<li><a href="components.html#_overview-2">12.1. Overview</a></li>
<li><a href="components.html#Component_Types">12.2. Component Types</a>
<ul class="sectlevel3">
<li><a href="components.html#_libraries">12.2.1. Libraries</a></li>
<li><a href="components.html#_runtime-environment-and-runtime-libraries">12.2.2. Runtime Environment and Runtime Libraries</a></li>
<li><a href="components.html#_tests">12.2.3. Tests</a></li>
<li><a href="components.html#_type-definitions">12.2.4. Type Definitions</a></li>
</ul>
</li>
<li><a href="components.html#package-json">12.3. Package.json File</a>
<ul class="sectlevel3">
<li><a href="components.html#_basic-properties">12.3.1. Basic Properties</a></li>
<li><a href="components.html#_n4js-properties">12.3.2. N4JS Properties</a></li>
<li><a href="components.html#_constraints">12.3.3. Constraints</a></li>
</ul>
</li>
<li><a href="components.html#_support-for-npm-scopes">12.4. Support for NPM Scopes</a></li>
<li><a href="components.html#sec:N4JS-Type-Definitions">12.5. N4JS Type Definitions</a>
<ul class="sectlevel3">
<li><a href="components.html#_specify-type-definition">12.5.1. Specify Type Definition</a></li>
<li><a href="components.html#_name-conventions">12.5.2. Name Conventions</a></li>
<li><a href="components.html#_version-conventions">12.5.3. Version Conventions</a>
<ul class="sectlevel4">
<li><a href="components.html#_define-a-new-type-definition-package">12.5.3.1. Define a New Type Definition Package</a></li>
<li><a href="components.html#_using-a-type-definition-package">12.5.3.2. Using a Type Definition Package</a></li>
<li><a href="components.html#_rational">12.5.3.3. Rational</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="components.html#_modules">12.6. Modules</a></li>
<li><a href="components.html#_api-and-implementation-component">12.7. API and Implementation Component</a>
<ul class="sectlevel3">
<li><a href="components.html#_execution-of-api-and-implementation-components">12.7.1. Execution of API and Implementation Components</a></li>
<li><a href="components.html#_api-and-implementation-with-di">12.7.2. API and Implementation With DI</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="#_plainjs">13. PlainJS</a>
<ul class="sectlevel2">
<li><a href="#_type-inference-and-validation-for-plain-js">13.1. Type Inference and Validation for Plain JS</a></li>
<li><a href="#_external-declarations">13.2. External Declarations</a>
<ul class="sectlevel3">
<li><a href="#_declaring-externals">13.2.1. Declaring externals</a></li>
<li><a href="#_instantiating-external-classes">13.2.2. Instantiating external classes</a></li>
<li><a href="#_implementation-of-external-declarations">13.2.3. Implementation of External Declarations</a></li>
<li><a href="#_example">13.2.4. Example</a></li>
</ul>
</li>
<li><a href="#_global-definitions">13.3. Global Definitions</a></li>
<li><a href="#_runtime-definitions">13.4. Runtime Definitions</a></li>
<li><a href="#_applying-polyfills">13.5. Applying Polyfills</a></li>
</ul>
</li>
<li><a href="jsdoc.html#_jsdoc-2">14. JSDoc</a>
<ul class="sectlevel2">
<li><a href="jsdoc.html#_general-n4jsdoc-features">14.1. General N4JSDoc Features</a>
<ul class="sectlevel3">
<li><a href="jsdoc.html#_provided-inline-tags">14.1.1. Provided Inline Tags</a>
<ul class="sectlevel4">
<li><a href="jsdoc.html#jsdoc_tag__code">14.1.1.1. @code</a></li>
<li><a href="jsdoc.html#jsdoc_tag__link">14.1.1.2. @link</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="jsdoc.html#_n4jsdoc-for-user-projects">14.2. N4JSdoc for User Projects</a>
<ul class="sectlevel3">
<li><a href="jsdoc.html#_standard-tags">14.2.1. Standard Tags</a>
<ul class="sectlevel4">
<li><a href="jsdoc.html#_-author">14.2.1.1. @author</a></li>
<li><a href="jsdoc.html#jsdoc_tag_param">14.2.1.2. @param</a></li>
<li><a href="jsdoc.html#jsdoc_tag_return">14.2.1.3. @return</a></li>
</ul>
</li>
<li><a href="jsdoc.html#_test-related-tags">14.2.2. Test Related Tags</a>
<ul class="sectlevel4">
<li><a href="jsdoc.html#jsdoc_tag__testee">14.2.2.1. @testee</a></li>
<li><a href="jsdoc.html#jsdoc_tag__testeeFromType">14.2.2.2. @testeeFromType</a></li>
<li><a href="jsdoc.html#_testeeType_and__testeeMember">14.2.2.3. @testeeType and @testeeMember</a></li>
<li><a href="jsdoc.html#jsdoc_tag_reqid_in_Tests">14.2.2.4. @reqid in Tests</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="jsdoc.html#_n4jsdoc-for-api-and-implementation-projects">14.3. N4JSDoc for API and Implementation Projects</a>
<ul class="sectlevel3">
<li><a href="jsdoc.html#jsdoc_tag__apiNote">14.3.1. @apiNote</a></li>
<li><a href="jsdoc.html#API_Project_Tags">14.3.2. API Project Tags</a>
<ul class="sectlevel4">
<li><a href="jsdoc.html#jsdoc_tag_apiState">14.3.2.1. @apiState</a></li>
</ul>
</li>
</ul>
</li>
</ul>
</li>
<li><a href="jsx.html#_jsx">15. JSX</a>
<ul class="sectlevel2">
<li><a href="jsx.html#_jsx-support">15.1. JSX Support</a></li>
<li><a href="jsx.html#_jsx-backend">15.2. JSX Backend</a></li>
</ul>
</li>
<li><a href="grammars.html#_grammars">16. Grammars</a>
<ul class="sectlevel2">
<li><a href="grammars.html#_type-expressions-grammar">16.1. Type Expressions Grammar</a></li>
<li><a href="grammars.html#_n4js-language-grammar">16.2. N4JS Language Grammar</a></li>
</ul>
</li>
<li><a href="jsobjects.html#_jsobjects">17. JSObjects</a>
<ul class="sectlevel2">
<li><a href="jsobjects.html#_object">17.1. Object</a></li>
<li><a href="jsobjects.html#_string">17.2. String</a></li>
<li><a href="jsobjects.html#_boolean">17.3. Boolean</a></li>
<li><a href="jsobjects.html#_number">17.4. Number</a>
<ul class="sectlevel3">
<li><a href="jsobjects.html#_static-attributes">17.4.1. Static Attributes</a></li>
</ul>
</li>
<li><a href="jsobjects.html#function">17.5. Function</a></li>
<li><a href="jsobjects.html#_error">17.6. Error</a></li>
<li><a href="jsobjects.html#_array">17.7. Array</a></li>
<li><a href="jsobjects.html#_date">17.8. Date</a></li>
<li><a href="jsobjects.html#_math">17.9. Math</a>
<ul class="sectlevel3">
<li><a href="jsobjects.html#static-attributes-1">17.9.1. Static Attributes</a></li>
</ul>
</li>
<li><a href="jsobjects.html#_regexp">17.10. RegExp</a></li>
<li><a href="jsobjects.html#_json">17.11. JSON</a></li>
</ul>
</li>
<li><a href="n4js_objects.html#_n4js-objects">18. N4JS Objects</a>
<ul class="sectlevel2">
<li><a href="n4js_objects.html#_reflection-model">18.1. Reflection Model</a></li>
<li><a href="n4js_objects.html#_error-types">18.2. Error Types</a>
<ul class="sectlevel3">
<li><a href="n4js_objects.html#_n4apinotimplemented">18.2.1. N4ApiNotImplemented</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="appendix_a_acronyms.html#_acronyms">Appendix A: Acronyms</a></li>
<li><a href="appendix_b_license.html#sec:License">Appendix B: License</a></li>
<li><a href="appendix_c_bibliography.html#_bibliography">Appendix C: Bibliography</a></li>
</ul>
</div>
</div>
<div id="content"><div class="sect1">
<h2 id="_plainjs"><a class="anchor" href="#_plainjs"></a><a class="link" href="#_plainjs">13. PlainJS</a></h2>
<div class="sectionbody">
<div class="paragraph">
<p>Since N4JS is a super set of JavaScript, is it both possible to use plain JavaScript in N4JS and vice versa.
There may be some obstacles due to concepts introduced by N4JS to make code more maintainable and robust:</p>
</div>
<div class="olist arabic">
<ol class="arabic">
<li>
<p>N4JS’ static type system may complain about some older JavaScript hacks. Declared types, in particular, are assumed to be implicitly frozen.</p>
</li>
<li>
<p>In N4JS, modules are used as namespaces with explicit export and import statements.
The notion of globals is not directly supported by N4JS as this leads to unexpected side effects (several components providing and thus overriding global definitions, for example).</p>
</li>
<li>
<p>N4JS defines a (ECMAScript 6 compatible) concept of object-oriented programming which may conflict with other plain JavaScript solutions.</p>
</li>
</ol>
</div>
<div class="paragraph">
<p>To overcome these problems, N4JS provides a couple of techniques summarized in this chapter.</p>
</div>
<div class="sect2 language-n4js">
<h3 id="_type-inference-and-validation-for-plain-js"><a class="anchor" href="#_type-inference-and-validation-for-plain-js"></a><a class="link" href="#_type-inference-and-validation-for-plain-js">13.1. Type Inference and Validation for Plain JS</a></h3>
<div class="paragraph">
<p>In plain JavaScript mode:</p>
</div>
<div class="olist arabic">
<ol class="arabic">
<li>
<p>All declared variables are inferred to <code>any+</code>.</p>
</li>
<li>
<p>All declared functions return and accept a variadic number of arguments of type <code>any+</code>.</p>
</li>
<li>
<p>It is allowed to use the <code>return</code> statement with or without an expression.</p>
</li>
<li>
<p>New expressions with a receiver of <code>any+</code> is allowed.</p>
</li>
<li>
<p>No type arguments are required for generic built-in types.</p>
</li>
<li>
<p>Assigning a value to a read-only variable is not checked.</p>
</li>
<li>
<p>Undeclared variables are treated as <code>any+</code> as well.</p>
</li>
</ol>
</div>
<div class="paragraph">
<p>Note that this essentially disables all validation particularly since methods such as the ’import’-like function <code>require</code> are unknown.</p>
</div>
</div>
<div class="sect2 language-n4js">
<h3 id="_external-declarations"><a class="anchor" href="#_external-declarations"></a><a class="link" href="#_external-declarations">13.2. External Declarations</a></h3>
<div class="paragraph">
<p>N4JS supports declaring external classes as a means to declare classes whose implementation is not N4JS so they can be used from N4JS.
Together with structural typing, this allows N4JS to seamlessly integrate frameworks and libraries which have not been implemented in N4JS but in plain ECMAScript or another language.</p>
</div>
<div class="openblock requirement">
<div class="content">
<div class="paragraph">
<p><a id="Req-IDE-163"></a><strong>Req. IDE-163:</strong> <a href="#Req-IDE-163">External allowed occurrences</a> (ver. 1)</p>
</div>
<div class="ulist">
<ul>
<li>
<p>Declarations with external flags are only allowed in files with the extension <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>n</mi><mn>4</mn><mi>j</mi><mi>s</mi><mi>d</mi></math> (so called N4JS definition files).</p>
</li>
<li>
<p>Only external classes, external interfaces marked with <code>@N4JS</code>, external enums, external function declarations and structurally typed interfaces are allowed in a <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>n</mi><mn>4</mn><mi>j</mi><mi>s</mi><mi>d</mi></math> file.</p>
</li>
<li>
<p>Declarations with external flags are allowed to subclass built-in type <code>Error</code> type and all of its descendants such as
<code>EvalError</code>, <code>RangeError</code>, <code>ReferenceError</code>, <code>SyntaxError</code>, <code>TypeError</code> and <code>URIError</code>, although any of the error types are annotated with <code>@N4JS</code>.</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>The following explanations apply to all external declarations except
where stated otherwise.</p>
</div>
<div class="paragraph">
<p>In general, an external declaration uses the same syntax as the declaration of a normal N4JS declaration with the addition of the modifier <code>external</code>.</p>
</div>
<div class="paragraph">
<p>External classifiers are always ’entirely external’ in that it is not possible to combine defined methods and external methods within a single class or interface.</p>
</div>
</div>
</div>
<div class="openblock requirement">
<div class="content">
<div class="paragraph">
<p><a id="Req-IDE-164"></a><strong>Req. IDE-164:</strong> <a href="#Req-IDE-164">External classes inheritance</a> (ver. 1)</p>
</div>
<div class="olist arabic">
<ol class="arabic">
<li>
<p>An external class <em>without</em> the <code>@N4JS</code> annotation can only inherit from another external class or from one of the built-in ECMAScript types (e.g. Object).
That is, by default external classes are derived from <code>Object</code>.</p>
</li>
<li>
<p>An external class <em>with</em> the annotation <code>@N4JS</code> can only inherit from another external class annotated with <code>@N4JS</code> or from non-external N4JS classes.</p>
</li>
</ol>
</div>
</div>
</div>
<div class="openblock requirement">
<div class="content">
<div class="paragraph">
<p><a id="Req-IDE-165"></a><strong>Req. IDE-165:</strong> <a href="#Req-IDE-165">Structurally typed interface implementation</a> (ver. 1)</p>
</div>
<div class="olist arabic">
<ol class="arabic">
<li>
<p>An external class <em>without</em> the annotation <code>@N4JS</code> can only be implemented by structurally typed interfaces.</p>
</li>
<li>
<p>An external class <em>with</em> the annotation <code>@N4JS</code> can only be implemented by structurally typed interfaces annotated with <code>@N4JS</code>.</p>
</li>
<li>
<p>An external interface <em>without</em> the annotation <code>@N4JS</code> must be defined structurally.</p>
</li>
</ol>
</div>
</div>
</div>
<div class="openblock requirement">
<div class="content">
<div class="paragraph">
<p><a id="Req-IDE-166"></a><strong>Req. IDE-166:</strong> <a href="#Req-IDE-166">External interface inheritance</a> (ver. 1)</p>
</div>
<div class="olist arabic">
<ol class="arabic">
<li>
<p>An interface in a n4jsd file <em>without</em> the annotation <code>@N4JS</code> can only inherit from another interface within a n4jsd file.</p>
</li>
<li>
<p>An interface <em>with</em> the <code>@N4JS</code> annotation can only inherit from another interface annotated with <code>@N4JS</code>.</p>
</li>
</ol>
</div>
</div>
</div>
<div class="openblock requirement">
<div class="content">
<div class="paragraph">
<p><a id="Req-IDE-167"></a><strong>Req. IDE-167:</strong> <a href="#Req-IDE-167">External class/interface members</a> (ver. 1)</p>
</div>
<div class="olist arabic">
<ol class="arabic">
<li>
<p>The static and instance methods, getters and setters of an external class must not have a method body.</p>
</li>
<li>
<p>The static and instance fields of an external class must not have an initializer.</p>
</li>
<li>
<p>The constructor of an external class without the annotation <code>@N4JS</code> must not be declared private.</p>
</li>
<li>
<p>Methods in interfaces with default implementation which cannot be expressed in the definition file must be annotated with <code>@ProvidesDefaultImplementation</code>.
This is only allowed in interfaces annotated with <code>@N4JS</code>.</p>
</li>
<li>
<p>Fields in interfaces or classes with initializers which cannot be expressed in the definition file, must be annotated with <code>@ProvidesInitializer</code>.
This is only allowed in classes or interfaces annotated with <code>@N4JS</code>.</p>
</li>
</ol>
</div>
<div class="paragraph">
<p>This means that in external classes, all members except constructors may be declared private even if the class is not annotated with <code>@N4JS</code>. In interfaces, however, private members are disallowed anyway,
cf. <a href="classifiers.html#Req-IDE-48">[Req-IDE-48]</a>.</p>
</div>
</div>
</div>
<div class="openblock requirement">
<div class="content">
<div class="paragraph">
<p><a id="Req-IDE-168"></a><strong>Req. IDE-168:</strong> <a href="#Req-IDE-168">Other external declarations</a> (ver. 1)</p>
</div>
<div class="olist arabic">
<ol class="arabic">
<li>
<p>The literals of an external enum must not have a value.</p>
</li>
<li>
<p>An external function declaration must not have a body.</p>
</li>
</ol>
</div>
</div>
</div>
<div class="sect3">
<h4 id="_declaring-externals"><a class="anchor" href="#_declaring-externals"></a><a class="link" href="#_declaring-externals">13.2.1. Declaring externals</a></h4>
<div class="paragraph">
<p>By default, the implicit supertype of an external class is Object.
If the <code>@N4JS</code> annotation is provided it is N4Object.
If a superclass is explicitly given, the constraints from the previous section apply.</p>
</div>
</div>
<div class="sect3">
<h4 id="_instantiating-external-classes"><a class="anchor" href="#_instantiating-external-classes"></a><a class="link" href="#_instantiating-external-classes">13.2.2. Instantiating external classes</a></h4>
<div class="paragraph">
<p>In most cases, it is desirable to instantiate external classes from external projects.
Publicly exporting the class definition and providing a public constructor is good practice.</p>
</div>
<div class="paragraph">
<p>In some cases, the instantiation from an outer scope is not wanted.
A possible approach is to use a structurally typed interface instead of a class to link to the implementation.</p>
</div>
<div class="paragraph">
<p>In case of API-definitions (see <a href="#_api-and-implementation-components">[_api-and-implementation-components]</a>), it might be useful to limit the visibility of classes to narrower scopes such as package or private.</p>
</div>
<div class="paragraph">
<p>External declarations can be instantiated if the following three requirements are fulfilled (not a constraint!):</p>
</div>
<div class="ulist">
<ul>
<li>
<p>External declarations have to be exported and be marked as public so they are accessible from outside.</p>
</li>
<li>
<p>The contained or inherited constructor of an external class must be public.</p>
</li>
<li>
<p>The external class must be linked to an implementation module (see below <a href="#_implementation-of-external-declarations">Implementation of External Declarations</a>).</p>
</li>
</ul>
</div>
</div>
<div class="sect3">
<h4 id="_implementation-of-external-declarations"><a class="anchor" href="#_implementation-of-external-declarations"></a><a class="link" href="#_implementation-of-external-declarations">13.2.3. Implementation of External Declarations</a></h4>
<div class="paragraph">
<p>All external declarations must be associated with an external implementation module in one way or another.
Any time the external declaration is imported, the compiler generates code that imports the corresponding implementation module at runtime.</p>
</div>
<div class="paragraph">
<p>There are two possible ways of linking an external declaration to its corresponding implementation:</p>
</div>
<div class="olist arabic">
<ol class="arabic">
<li>
<p>By naming convention defined in the <code>package.json</code> file.</p>
</li>
<li>
<p>By declaring that the implementation is provided by the JavaScript runtime, see <a href="#_runtime-definitions">Runtime Definitions</a> for details.</p>
</li>
</ol>
</div>
<div class="paragraph">
<p>The naming convention is based on the <code>external</code> source fragments defined in the <code>package.json</code> file (<a href="components.html#package-json">Package.json File</a>).
If the implementation is provided by the runtime directly, then this can be also specified in the <code>package.json</code> file by a module filter.</p>
</div>
<div class="paragraph">
<p>The implicit link via the naming convention is used to link an external class declaration to its non-N4JS implementation module.
It does not effect validation, but only compilation and runtime.
Essentially, this makes the compiler generate code so that at runtime, the linked implementation module is imported instead of the declaration module.</p>
</div>
<div class="paragraph">
<p>In most use cases of external declarations you also want to disable validation and module wrapping by specifying appropriate filters in the <code>package.json</code> file.</p>
</div>
<div class="paragraph">
<p>Occasionally it is not possible for the validation to correctly detect a corresponding implementation element.
For that reason, it is possible to disable validation of implementations completely via <code>@@IgnoreImplementation</code>.</p>
</div>
<div class="openblock requirement">
<div class="content">
<div class="paragraph">
<p><a id="Req-IDE-169"></a><strong>Req. IDE-169:</strong> <a href="#Req-IDE-169">Implementation of External Declarations</a> (ver. 1)</p>
</div>
<div class="paragraph">
<p>For a given external declaration <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>D</mi></math> but not for
API-definitions <sup class="footnote">[<a id="_footnoteref_60" class="footnote" href="appendix_c_bibliography.html#_footnote_60" title="View footnote.">60</a>]</sup>, the
following constraints must hold:</p>
</div>
<div class="olist arabic">
<ol class="arabic">
<li>
<p>If the declaration is neither provided by runtime nor validation of implementation is disabled,
a corresponding implementation must be found by the naming convention.
If no such implementation is found, a warning is generated.</p>
</li>
</ol>
</div>
</div>
</div>
<div id="external-definitions-and-implementations" class="exampleblock">
<div class="title">Example 105. External Definitions and Their Implementations</div>
<div class="content">
<div class="paragraph">
<p>If, in addition to standard <code>source</code>, the <code>source-external</code> fragment is provided in <code>Sources</code>, <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>n</mi><mn>4</mn><mi>j</mi><mi>s</mi><mi>d</mi></math> files in the folder tree below source folders
will be related to modules of the same name in the external folders. This is shown in <a href="#fig-external-class-naming">External Class Implementation, Naming Convention</a>.</p>
</div>
<div id="fig-external-class-naming" class="imageblock" style="text-align: center">
<div class="content">
<img src="chapters/13_plainJS/fig/externalClassImplementation_naming.svg" alt="externalClassImplementation naming">
</div>
<div class="title">Figure 16. External Class Implementation, Naming Convention</div>
</div>
</div>
</div>
</div>
<div class="sect3">
<h4 id="_example"><a class="anchor" href="#_example"></a><a class="link" href="#_example">13.2.4. Example</a></h4>
<div class="paragraph">
<p>Assume the following non-N4JS module:</p>
</div>
<div id="ex:External_Classes_Example" class="exampleblock">
<div class="title">Example 106. External Classes</div>
<div class="content">
<div class="listingblock">
<div class="content">
<pre class="highlight"><code class="language-javascript" data-lang="javascript">module.exports = {
    "Point": function Point(x, y) {
        this.x = x;
        this.y = y;
    },

    "Circle": function Circle(center, radius) {
        this.center = center;
        this.radius = radius;
        this.scaleX = function(x){ this.x = x; }
    this.scaleY= function(y){ this.y = y; }
    }
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>Assuming</p>
</div>
<div class="ulist">
<ul>
<li>
<p><code>shapes.js</code> is placed in project folder <strong>/external/a/b/shapes.js</strong></p>
</li>
<li>
<p><code>shapes.n4jsd</code> is placed in project folder <strong>/src/a/b/shapes.n4jsd</strong></p>
</li>
<li>
<p><code>package.json</code> defines <strong>src</strong> as source folder and <strong>external</strong> as external source folder</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>the following N4JS external class declarations in <strong>shapes.n4jsd</strong> are sufficient:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code class="language-javascript" data-lang="javascript">export external public class Point {
    x: number; y: number;
    constructor(x: number, y: number);
}

export external public class Circle {
    center: Point; radius: number;
    constructor(center: Point, radius: number);
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>Note that the class and interface names in n4jsd files must match those in the js files, respectively.</p>
</div>
</div>
</div>
<div class="exampleblock">
<div class="title">Example 107. Structurally-typed external interfaces</div>
<div class="content">
<div class="listingblock">
<div class="content">
<pre class="highlight"><code class="language-javascript" data-lang="javascript">export external public interface ~Scalable {
    scaleX(factor: number);
    scaleY(factor: number);
}

export external public class Circle implements Scalable {
    center: Point;
    radius: number; x: number; y: number;

    @Override public scaleX(factor: number);
    @Override public scaleY(factor: number);

    constructor(center: Point, radius: number);
}</code></pre>
</div>
</div>
</div>
</div>
</div>
</div>
<div class="sect2 language-n4js">
<h3 id="_global-definitions"><a class="anchor" href="#_global-definitions"></a><a class="link" href="#_global-definitions">13.3. Global Definitions</a></h3>
<div class="paragraph">
<p>Existing JavaScript libraries and built-in objects provided by certain JavaScript environments often globally define variables.
Although it is not recommended to use global definitions, this cannot always be avoided.</p>
</div>
<div class="paragraph">
<p>N4JS supports global definitions via the annotation <code>Global</code>.
This annotation can only be defined on modules (via <code>@@Global</code>) – this means that all declarations in the module are
globally defined.<sup class="footnote">[<a id="_footnoteref_61" class="footnote" href="appendix_c_bibliography.html#_footnote_61" title="View footnote.">61</a>]</sup></p>
</div>
<div class="paragraph">
<p>We introduce a new pseudo property <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>g</mi><mi>l</mi><mi>o</mi><mi>b</mi><mi>a</mi><mi>l</mi></math> on all declared elements accordingly:</p>
</div>
<div class="dlist">
<dl>
<dt class="hdlist1"><code>@Global</code> </dt>
<dd>
<p>Boolean flag set to true if annotation <code>@Global</code> is set in containing module. The flag indicates that the exported element is globally available and must not be imported.</p>
</dd>
</dl>
</div>
<div class="paragraph">
<p>Since definition of global elements is not supported by N4JS directly, this can be only used in external definitions.
A declaration with <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>g</mi><mi>l</mi><mi>o</mi><mi>b</mi><mi>a</mi><mi>l</mi></math> can be used without explicit import statement. It is not possible to import these declarations.</p>
</div>
<div class="openblock requirement">
<div class="content">
<div class="paragraph">
<p><a id="Req-IDE-170"></a><strong>Req. IDE-170:</strong> <a href="#Req-IDE-170">Global Definitions</a> (ver. 1)</p>
</div>
<div class="paragraph">
<p>Global Definitions</p>
</div>
<div class="paragraph">
<p>For a declaration <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>D</mi></math> with <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>D</mi><mo>.</mo><mi>g</mi><mi>l</mi><mi>o</mi><mi>b</mi><mi>a</mi><mi>l</mi><mo>=</mo><mstyle mathvariant="monospace"><mtext>true</mtext></mstyle></math>, not a polyfill (<math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>D</mi><mo>.</mo><mi>p</mi><mi>o</mi><mi>l</mi><mi>y</mi><mi>f</mi><mi>i</mi><mi>l</mi><mi>l</mi><mo>=</mo><mstyle mathvariant="monospace"><mtext>false</mtext></mstyle><mo>)</mo></mrow></math>, the following constraints must hold:</p>
</div>
<div class="olist arabic">
<ol class="arabic">
<li>
<p>The name of the definition must not be equal to any primitive type (<code>string</code>, <code>number</code> etc.), <code>any</code>, or an built-in N4 type (<code>N4Object</code> etc.).</p>
</li>
<li>
<p>If the name of the definition equals a basic runtime time Object Type then the project must be a runtime environment:</p>
</li>
</ol>
</div>
<div class="paragraph">
<p><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>D</mi><mo>.</mo><mi>n</mi><mi>a</mi><mi>m</mi><mi>e</mi><mo>∈</mo><mrow><mo>{</mo></mrow></math>
<math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mspace width="3.0em"/><mi>'</mi></msup><mi>O</mi><mi>b</mi><mi>j</mi><mi>e</mi><mi>c</mi><msup><mi>t</mi><mi>'</mi></msup><msup><mo>,</mo><mi>'</mi></msup><mi>F</mi><mi>u</mi><mi>n</mi><mi>c</mi><mi>t</mi><mi>i</mi><mi>o</mi><msup><mi>n</mi><mi>'</mi></msup><msup><mo>,</mo><mi>'</mi></msup><mi>A</mi><mi>r</mi><mi>r</mi><mi>a</mi><msup><mi>y</mi><mi>'</mi></msup><msup><mo>,</mo><mi>'</mi></msup><mi>S</mi><mi>t</mi><mi>r</mi><mi>i</mi><mi>n</mi><msup><mi>g</mi><mi>'</mi></msup><msup><mo>,</mo><mi>'</mi></msup><mi>B</mi><mi>o</mi><mi>o</mi><mi>l</mi><mi>e</mi><mi>a</mi><msup><mi>n</mi><mi>'</mi></msup></math>
<math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mspace width="3.0em"/><mi>'</mi></msup><mi>N</mi><mi>u</mi><mi>m</mi><mi>b</mi><mi>e</mi><msup><mi>r</mi><mi>'</mi></msup><msup><mo>,</mo><mi>'</mi></msup><mi>M</mi><mi>a</mi><mi>t</mi><msup><mi>h</mi><mi>'</mi></msup><msup><mo>,</mo><mi>'</mi></msup><mi>D</mi><mi>a</mi><mi>t</mi><msup><mi>e</mi><mi>'</mi></msup><msup><mo>,</mo><mi>'</mi></msup><mi>R</mi><mi>e</mi><mi>g</mi><mi>E</mi><mi>x</mi><msup><mi>p</mi><mi>'</mi></msup><msup><mo>,</mo><mi>'</mi></msup><mi>E</mi><mi>r</mi><mi>r</mi><mi>o</mi><msup><mi>r</mi><mi>'</mi></msup><msup><mo>,</mo><mi>'</mi></msup><mi>J</mi><mi>S</mi><mi>O</mi><msup><mi>N</mi><mi>'</mi></msup></math>
<math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo>}</mo></mrow></math>
<math xmlns="http://www.w3.org/1998/Math/MathML"><mo>⇒</mo><mi>D</mi><mo>.</mo><mi>c</mi><mi>o</mi><mi>n</mi><mi>t</mi><mi>a</mi><mi>i</mi><mi>n</mi><mi>i</mi><mi>n</mi><mi>g</mi><mi>P</mi><mi>r</mi><mi>o</mi><mi>j</mi><mi>e</mi><mi>c</mi><mi>t</mi><mo>.</mo><mi>t</mi><mi>y</mi><mi>p</mi><mi>e</mi><mo>=</mo><mstyle mathvariant="monospace"><mtext>runtimeEnvironment</mtext></mstyle></math></p>
</div>
</div>
</div>
</div>
<div class="sect2 language-n4js">
<h3 id="_runtime-definitions"><a class="anchor" href="#_runtime-definitions"></a><a class="link" href="#_runtime-definitions">13.4. Runtime Definitions</a></h3>
<div class="paragraph">
<p>Some elements are predefined by the JavaScript runtime such as DOM elements by the browser or built-in ECMAScript or non-standard objects.
These elements can be defined by means of external definitions; however, no actual implementation can be provided as these elements are actually provided by the runtime itself.</p>
</div>
<div class="paragraph">
<p>Since these cases are rather rare and in order to enable additional checks such as verification that a given runtime actually provides the elements,
this kind of element can only be defined in components of type runtime environment or runtime library (cf <a href="components.html#_runtime-environment-and-runtime-libraries">Runtime Environment and Runtime Libraries</a>).</p>
</div>
<div class="paragraph">
<p>N4JS supports runtime definitions via the annotation <code>@ProvidedByRuntime</code>.
This annotation can be defined</p>
</div>
<div class="ulist">
<ul>
<li>
<p>on modules (via <code>@@ProvidedByRuntime</code>)– this means that all declarations in the module are provided by the runtime</p>
</li>
<li>
<p>on export statements or declarations.</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>We introduce a new pseudo property <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>p</mi><mi>r</mi><mi>o</mi><mi>v</mi><mi>i</mi><mi>d</mi><mi>e</mi><mi>d</mi><mi>B</mi><mi>y</mi><mi>R</mi><mi>u</mi><mi>n</mi><mi>t</mi><mi>i</mi><mi>m</mi><mi>e</mi></math> accordingly:</p>
</div>
<div class="dlist">
<dl>
<dt class="hdlist1"><code>@ProvidedByRuntime</code> </dt>
<dd>
<p>Boolean flag set to true if the annotation <code>@ProvidedByRuntime</code> is set.
Flag indicates that the element is only declared in the module but its implementation is provided by the runtime.</p>
</dd>
</dl>
</div>
<div class="paragraph">
<p>Since built-in types are usually defined globally, the annotation <code>@ProvidedByRuntime</code> is usually used in combination with <code>@Global</code>.</p>
</div>
<div class="openblock requirement">
<div class="content">
<div class="paragraph">
<p><a id="Req-IDE-171"></a><strong>Req. IDE-171:</strong> <a href="#Req-IDE-171">Provided By Runtime</a> (ver. 1)</p>
</div>
<div class="paragraph">
<p>For a declaration <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>D</mi></math> with <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>D</mi><mo>.</mo><mi>p</mi><mi>r</mi><mi>o</mi><mi>v</mi><mi>i</mi><mi>d</mi><mi>e</mi><mi>d</mi><mi>B</mi><mi>y</mi><mi>R</mi><mi>u</mi><mi>n</mi><mi>t</mi><mi>i</mi><mi>m</mi><mi>e</mi><mo>=</mo><mstyle mathvariant="monospace"><mtext>true</mtext></mstyle></math>, the following constraints must hold:</p>
</div>
<div class="olist arabic">
<ol class="arabic">
<li>
<p>The declaration must either be an export declaration itself or an exportable declaration.</p>
</li>
<li>
<p>The declaration must be contained in a definition module.</p>
</li>
<li>
<p>The declaration must be (indirectly) contained in a component of type <math xmlns="http://www.w3.org/1998/Math/MathML"><mstyle mathvariant="monospace"><mtext>runtimeEnvironment</mtext></mstyle></math> or <math xmlns="http://www.w3.org/1998/Math/MathML"><mstyle mathvariant="monospace"><mtext>runtimeLibrary</mtext></mstyle></math>.</p>
</li>
<li>
<p>There must be no implementation file with the same name as the definition module if annotation is defined for a whole module.</p>
</li>
</ol>
</div>
</div>
</div>
</div>
<div class="sect2 language-n4js">
<h3 id="_applying-polyfills"><a class="anchor" href="#_applying-polyfills"></a><a class="link" href="#_applying-polyfills">13.5. Applying Polyfills</a></h3>
<div class="paragraph">
<p>(Runtime) Libraries often do not provide completely new types but modify existing types.
The ECMA-402 Internationalization Standard [<a href="appendix_c_bibliography.html#ECMA12a">ECMA12a</a>], for example, changes methods of the built-in class <code>Date</code> to be timezone-aware.
Other scenarios include new functionality provided by browsers which are not part of an official standard yet.
Even ECMAScript 6 [<a href="appendix_c_bibliography.html#ECMA15a">ECMA15a</a>] extends the predecessor [<a href="appendix_c_bibliography.html#ECMA11a">ECMA11a</a>] in terms of new methods or new method parameters added to existing types.
It also adds completely new classes and features, of course.</p>
</div>
<div class="paragraph">
<p>The syntax of runtime polyfills is described in section <a href="extended_fetaures.html#_polyfill-definitions">Polyfill Definitions</a>.
Here, an example of applying a runtime polyfill is detailed.</p>
</div>
<div class="exampleblock">
<div class="title">Example 108. Object.observe with Polyfill</div>
<div class="content">
<div class="paragraph">
<p>The following snippet demonstrates how to define a polyfill of the built-in class <code>Object</code> to add the new ECMAScript 7 observer functionality.
This snippet has to be defined in a runtime library or environment.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code class="language-n4js" data-lang="n4js">@@ProvidedByRuntime
@@Global

@Polyfill
export external public class Object extends Object {
    public static Object observe(Object object, Function callback, Array&lt;string&gt;? accept);

}</code></pre>
</div>
</div>
<div class="paragraph">
<p>A client referring to this runtime library (or environment) can now access the observer methods as if it were defined directly in the original declaration of <code>Object</code>.</p>
</div>
</div>
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